1. Field of the Invention
The present invention relates to an optical fiber suitable as an optical transmission line or dispersion compensator.
2. Related Background Art
The following optical fibers have conventionally been known, for example. A microstructured optical fiber disclosed in Japanese Patent Application Laid-Open No. HEI 10-95628 has a core region, which is usually solid, surrounded by a cladding region that comprises a multiplicity of spaced apart cladding features that are elongate in the axial direction and disposed in a first cladding material. The core region has an effective diameter d0 and an effective refractive index N0. Each cladding feature has a refractive index that differs from that of the first cladding material, and the cladding region has an effective refractive index that is less than N0. Further, it is disclosed that a large dispersion is obtained since the cladding region comprises an inner cladding region having an effective refractive index Ncl and an outer cladding region having an effective refractive index Nco (where Ncl less than Nco).
OFC""96 Technical Digest, ThA3 discloses an optical fiber having a W-shaped refractive index profile, and discloses that a low chromatic dispersion (with a large negative value) can be realized in this optical fiber.
Electronics Letters, vol. 18, pp. 824-826 (1982) discloses that, by being provided with xe2x80x9cside tunnelsxe2x80x9d on both sides of a core region, not only high normalized birefringence is realized, but also the cutoff frequency difference between two polarization modes is enlarged, whereby an absolutely single-polarization optical fiber can be realized.
U.S. Pat. No. 5,907,652 discloses the following air-clad optical fiber. Namely, it is a silica-based optical fiber comprising, successively from the fiber center toward the outer periphery, a core region, an inner cladding region, a first outer cladding region, and a second outer cladding region, in which the refractive index of inner cladding is less than that of the core region, and the effective refractive index of first outer cladding region is less than 1.35. Also, the first outer cladding region is selected such that optical characteristics of the optical fiber do not depend on the second outer cladding region. It discloses that the air-clad optical fiber is suitable for cladding-pumped optical fiber lasers and long-period fiber gratings.
In the microstructured optical fiber disclosed in JP 10-95628A, however, microstructures are distributed over the whole cladding, whereby the number of microstructures is large. For example, the above-mentioned publication states that xe2x80x9cOur simulations indicate that at least 4 layers of second capillary features should be provided.xe2x80x9d In this case, the number of capillary features would be at least 90, thus becoming large. If the number of microstructures is large as such, then the making becomes difficult. According to the above-mentioned publication, the process of making the microstructured optical fiber is as follows. Namely, silica capillary tubes and a silica rod with no bore are prepared, a tube bundle is formed by arranging a number of silica tubes around the silica rod, the tube bundle and an over cladding tube are collapsed so as to yield a preform, and then an optical fiber is drawn from this preform. However, it takes time and effort to form a tube bundle by arranging small-diameter silica tubes into a bundle without disorder. Also, since there is a strong possibility of the arrangement being disordered, the making with a favorable reproducibility is hard to achieve. The making becomes more difficult as the number of microstructures increases.
On the other hand, a step of boring a preform of a conventional impurity-doped type optical fiber by use of a boring device may be used instead of the above-mentioned making process. Even in the case using this step, however, the conventional microstructured optical fiber has a number of microstructures, whereby the cost of manufacture becomes high.
Also, the optical fiber disclosed in the above-mentioned publication has problems as follows in particular when the microstructures are bores. First, the mechanical strength of optical fiber is lowered due to the bores included therein, whereby strengths against tension and lateral pressures may decrease. Second, there is a possibility of absorption loss occurring due to OH group on surfaces of bores and water vapor within the bores. Therefore, during operations of making or fiber connection, a treatment for lowering the possibility of water vapor entering the bores is necessary, which makes the operations difficult. Third, if glass melts upon fusion splicing and thereby closes bores, then the effective refractive index difference between the core and cladding is lost, so that the optical power leaking out into the cladding remarkably increases, whereby propagation loss becomes greater at the fused part. The first and second problems become more influential as the number of microstructures increases.
On the other hand, the refractive index difference realizable in the impurity-doped type optical fiber disclosed in OFC""96 Technical Digest, ThA3 is small. As a result, realizable value ranges are restricted in terms of the magnitude of absolute value of negative dispersion, magnitude of absolute value of negative dispersion slope, size of effective core area, and reduction of bending loss.
The optical fiber disclosed in Electronics Letters, vol. 18, pp. 824-826 (1982) yields a large linear birefringence with xe2x80x9cside tunnelsxe2x80x9d of air provided on both sides of its core. However, it is desirable that birefringence be smaller for optical transmission application, such as those in which the optical fiber is incorporated in a part of an existing optical transmission line in particular. If the polarization state of light incident on an optical fiber having a large birefringence does not match either of the principal polarization states of fiber, then transmission quality deteriorates due to polarization mode dispersion. Hence, a device for making the polarization state of incident light constant is necessary, which raises the cost. Also, most of existing optical transmission lines have no polarization selectivity, whereby the polarization state of light emitted therefrom is not constant. Thus, the polarization state of light having an inconstant polarization state is hard to keep constant.
In the air-clad optical fiber disclosed in U.S. Pat. No. 5,907,652, a chromatic dispersion having a large negative value and a chromatic dispersion slope having a large negative value are hard to obtain. This is because of the fact that this optical fiber is mainly aimed at lowering the effective refractive index of first outer cladding region, so as to prevent the second outer cladding region from influencing optical characteristics.
In view of such circumstances, it is an object of the present invention to provide an optical fiber which can realize a low chromatic dispersion (having a large negative value), a low chromatic dispersion slope(having a large negative value), a large effective core area, and a low bending loss. It is another object of the present invention to provide an optical fiber facilitating its making, cutting down its cost, improving its strengths against tension and lateral pressures, lowering the possibility of absorption loss occurring due to OH group on surfaces of bores and water vapor within bores, and reducing power loss at fusion splice.
For satisfying the above-mentioned objects, the present invention provides an optical fiber comprising a core region constituted by a substantially homogeneous medium; an inner cladding region surrounding the core region; and an outer cladding region, constituted by a substantially homogeneous medium, surrounding the inner cladding region; wherein the core region, inner cladding region, and outer cladding region are regions extending along a fiber axis and influencing optical characteristics; wherein an average refractive index n0 of the core region, an average refractive index n1 of the inner cladding region, and an average refractive index n2 of the outer cladding region satisfy the relationship of n1 less than n2 less than n0; and wherein the inner cladding region includes at least three microstructures each extending along the fiber axis and comprising an auxiliary medium having a refractive index different from that of a main medium constituting the inner cladding region.
Within a cross section perpendicular to the fiber axis, the core region has a substantially circular form, whereas the inner and outer cladding regions have substantially annular forms. The average refractive index of each of the core region, inner cladding region, and outer cladding region can be given by the following navg:                               n          avg                =                              [                                          1                                  π                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  b                        2                                            -                                              a                        2                                                              )                                                              ⁢                              xe2x80x83                            ⁢                                                ∫                  a                  b                                ⁢                                                      n                    2                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                          r                      ,                      θ                                        )                                    ⁢                                      xe2x80x83                                    ⁢                  r                  ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    θ                                    ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    r                                                                        ]                                1            2                                              (        1        )            
where a is the inner radius of the region (0 in the case of core region), b is the outer radius, r and xcex8 are polar coordinates representing a position within a fiber cross section, and n(r, xcex8) is a refractive index distribution within the cross section. In general, the average refractive indices in core region, inner cladding region, and outer cladding region vary depending on the respective definitions of regions. The expressions xe2x80x9ccomprising a core region constituted by a substantially homogeneous medium; an inner cladding region surrounding the core region; and an outer cladding region, constituted by a substantially homogeneous medium, surrounding the inner cladding regionxe2x80x9d and xe2x80x9can average refractive index n0 of the core region, an average refractive index n1 of the inner cladding region, and an average refractive index n2 of the outer cladding region satisfy the relationship of n1 less than n2 less than n0xe2x80x9d mean that there is such a way of defining the core region, inner cladding region, and outer cladding region as to satisfy the above-mentioned inequality. For improving the fiber strength, the outer cladding region may be surrounded with a jacket region made of a material such as glass or resin. Here, it is necessary for the outer cladding region to have a sufficient radial thickness in order to prevent the jacket region from influencing the optical characteristic. On the other hand, the outer cladding region is a region influencing the optical characteristic, whereby the average refractive index and thickness of the inner cladding region are selected such that the outer cladding region influences the optical characteristic.
Each of the core region and outer cladding region is constituted by a substantially homogeneous medium. It means that the main ingredient of the material constituting each of these regions is same in the respective region. Here, a configuration in which the impurity concentration is varied within the region can be employed when appropriate. For example, the core region may be silica glass containing Ge as an impurity while employing a structure in which Ge concentration decreases from the center toward the outer periphery.
The main medium is a medium which can actually constitute the optical fiber by itself. A plurality of main medium regions which are not connected together must not exist in a single optical fiber. On the other hand, the auxiliary medium may be a medium which cannot actually constitute the optical fiber by itself. A plurality of auxiliary medium regions disconnected from each other may exist in a single optical fiber. A typical example of the main medium is silica-based glass, whereas typical examples of the auxiliary medium are gases or liquids.
Thus, in addition to a main medium constituting the inner cladding region, microstructures constituted by an auxiliary medium having a refractive index different from that of the main medium are provided in the inner cladding region of the optical fiber in accordance with the present invention. On the other hand, the outer cladding region is constituted by a substantially homogeneous medium and includes no microstructures. This is based on the inventor""s discovery that, for yielding a favorable characteristic such as a dispersion having a large negative value in an optical fiber in which the average refractive index of the inner cladding region is less than that of the outer cladding region, it will be sufficient if the average refractive index of the inner cladding region is lowered by providing microstructures therein without necessitating providing microstructures in the outer cladding region. On the other hand, by being provided with microstructures each of which consists of an auxiliary medium having a refractive index less than that of the main medium, the average refractive index of inner cladding region can be made much lower than that in the case without the microstructures. As a result, favorable characteristics such as a dispersion with a larger negative value, a dispersion slope with a larger negative value, a larger effective core area, and a smaller bending loss can be obtained as compared with the conventional impurity-doped type optical fiber. Also, unlike the air-clad optical fiber, the optical fiber of the present invention can realize a dispersion with a large negative value and a dispersion slope with a large negative value. This is because of the fact that the outer cladding region surrounding the inner cladding region including microstructures influences optical characteristics, such as chromatic dispersion characteristics in particular. Further, since the outer cladding region is constituted by a substantially homogeneous medium and includes no microstructures, the number of microstructures to be provided can be drastically reduced compared to that in the conventional microstructured optical fiber. As a result, the optical fiber can easily be made with a favorable reproducibility using any of the method in which silica tubes are arranged and the method in which a preform is bored by use of a boring device, whereby the cost of manufacture can be cut down.
In the case where the microstructures comprise bores in particular, strengths against tension and lateral pressures improve as compared with the conventional microstructured optical fiber when the number of microstructures decreases, and the making and connecting become easier since the probability of absorption loss occurring due to OH group on surfaces of bores and water vapor within the bores decreases. Further, since the refractive index of core region is higher than that of outer cladding region, the waveguiding function of the fiber will not be lost even if bores are closed in the inner cladding, whereby attenuation due to fusion splice can be reduced.